Anatomy of the Sacroiliac Joint (SI) The sacroiliac (SI) joint is formed where your sacrum—the bony plate at the back of your pelvis—meets your ilium, the elephant ears of your pelvis. The SI joint articulates just a few degrees and mostly wants to stay in place, like a closed door. Unlike your spine, which can flex, extend, and rotate, your SI joint has very little movement. When it moves more than it should, it can cause problems related to instability and joint laxity, rather than stiffness.